This invention relates generally to a method for surface treatment, and an ornament and electronic device, and in particular to applying a transparent protective layer on the surface of titanium or titanium alloys.
As described in Japanese Unexamined Utility Model Publication No. 56-74060, Japanese Unexamined Patent Publication No. 58-96869 and Japanese Unexamined Patent Publication No. 62-263977, methods for coating a metal surface, such as aluminum, with a rigid transparent coating film thereby adding corrosion resistant, abrasion resistant and impact resistant properties to the metal surface, are known in the art. The transparent coating film is generally composed of a variety of organic resins and inorganic substances.
In one surface coating technique, a transparent coating layer of rigid inorganic materials including aluminum oxide or aluminum silicon is conventionally coated on the surface of metal facing cases such as those used for watches and clocks.
In the case of wristwatches, machine works are generally applied to the surface of facing cases constructed from various kind of metals such as stainless steel, brass and other metals. Examples of such machine works are honing processing by blasting fine grains, nicking processing and mirror finishing by buff finishing or barrel finishing. When such machine works are used, surface adhesion substances such as oxide layers are very likely to form on the surface of the facing cases, thus greatly diminishing the quality of appearance of the wristwatch.
Recently, titanium has been identified as a light and highly corrosion resistant material having a preventive effect for metal allergy. Often, titanium is used in combination with other metals where titanium is the main component thus forming a titanium alloy. Examples of metals that may be combined with titanium are Al, V, Mo, W, Fe, Co, Cr, Cu, Ag, Pt, Pd and Zn. The content of these metals is not especially limited as long as the metals do not alter the intrinsic properties of titanium.
Because titanium and titanium alloys oxidize easily, components constructed from these materials often have a titanium oxide layer (color change layer) formed on their surface. Formation of this oxide layer becomes evident when the machine works are applied to the component. The presence of an oxide layer can blacken the surface, which deterioates the quality of appearance of the component and therefore should be avoided.
The facing case of a watch is produced by assembling facing members of watches such as a case body, rear cover, cover glass and the like. A bezel is provided around the periphery of the cover glass (water crystal) in some watches. At least one of the case body or bezel of the facing members of watches may be produced with titanium or titanium alloys.
The facing members of a wristwatch are often produced by press molding, casting, powder metallurgy, lost wax and metal injection molding (MIM) techniques using titanium or a titanium alloy as a starting material. An appropriate cutting process can be applied to the facing member produced by press molding, if required.
Although titanium has advantages, problems can also exist when titanium is used for the facing case of a wristwatch. Although honing processing (honing finish) is the most frequently used machine works for finishing the surface of a facing case constructed from titanium, the surface of the titanium layer is easily damaged and oxidized when bombarded with glass beads during honing processing. This can blacken the surface. Also, because honing processing causes the surface to become uneven, the surface can be easily contaminated with fingerprints that are difficult to wipe off. In addition, when titanium is used for wristwatches, the wristwatches are more easily scratched because titanium has a Vickers hardness of only 150, which is less than the Vickers hardness of 200 for stainless steel (SUS).
Accordingly, it is desirable to provide a method and product which overcome drawbacks of the prior art.